1. Field of the Invention
This invention relates to the recovery of hydrocarbons from subterranean formations through the use of microemulsions, and to microemulsion compositions. In an other aspect this invention relates to the recovery of hydrocarbons from subterranean formations by utilizing optimal microemulsions comprising a hydrocarbon or hydrocarbon mixture, an aqueous solution, and at least one surfactant. In still another aspect this invention relates to microemulsions from the single phase region in the ternary diagram of an optimal microemulsion system at the reservoir salinity, that are useful in the recovery of hydrocarbons from a subterranean formation wherein the microemulsion comprises a hydrocarbon or hydrocarbon mixture tailored for the formation based on viscosity criteria, an aqueous solution, and at least one surfactant.
2. Description of the Related Art
Hydrocarbons are produced from subterannean formations by a variety of methods. "Primary recovery" techniques refer to those techniques that utilize only the initial formation energy to recover the hydrocarbons in the subterranean formation, and include natural flow, gas lifting, gas pressurization, and pumping methods. However it has long been known in the petroleum industry that primary recovery techniques are only capable of producing a small fraction of the original oil in place in the reservoir. Consequently there have been developed the so called "secondary and tertiary recovery" techniques, or "enhanced oil recovery" ("EOR") techniques, which have as their primary purpose the recovery of additional quantities of hydrocarbons known to be present in the reservoir, after the naturally occurring forces have declined in their ability to expel oil.
One of the most economical and perhaps most practiced of the EOR techniques is waterflooding. In waterflooding, an aqueous solution is injected into the reservoir through one or more injection wells to drive the hydrocarbons to one or more offset production wells. However, even after a typical waterflood the reservoir may retain a great portion of its original oil in place. It is well known that much of the retained oil in the reservoir after a typical waterflood is in the form of discontinuous globules or discrete droplets which are trapped in the pore spaces of the reservoir. The high normal interfacial tension between the reservoir water and oil prevents these discrete droplets from deforming to pass through narrow constrictions in the pore channels. Consequently, surface-active agents or surfactants have been added to the flood water solution to lower the interfacial tension between the water and the oil and thereby allow the oil droplets to deform and flow with the injected flood water. It is generally conceded that the interfacial tension between the oil and water must be reduced from the normal reservoir interfacial tension which is on the order of about 20 dyne/cm to less than 0.1 dyne/cm for effective recovery.
However, as effective as conventional surfactant waterflooding may be in recovering a portion of the remaining oil from subterranean formations, it is not without its shortcomings which detract seriously from its value. In general the major drawback with the use of surfactants in general is the tendency of the surfactants to be deleted from the injected waterflood solution. This depletion is thought to occur in at least one of several ways. For example, it is possible that at least a portion of the surface-active agents or surfactants may be adsorbed on the porous surface of the reservoir, or physically entrapped within the pore spaces of the reservoir matrix. It is also known that many surfactants react with ionic substances in the formation and are precipitated. Such depleted surfactants, whether adsorbed, entrapped, reacted or precipitated are unable to interact at the oil/water interface to reduce the interfacial tension. Consequently the oil recovery efficiency of the waterflood is reduced due to the surfactant depletion.
One method for reducing surfactant depletion and increasing the efficiency of the recovery is by the use of microemulsions. Microemulsions are well known (see for example U.S. Pat. No. 3,254,714, Gogarty et al, issued June 1966, and U.S. Pat. No. 3,981,361 Healy, issued Sep. 21, 1976) and are mixtures of oil, water, and a surfactant.
FIGS. 1, 2 and 3 show phase diagrams for water-oil-surfactant systems and the effect of salinity on phase behavior. Healy et al, in "Multiphase Microemulsion Systems", discloses that microemulsion floods conducted at "optimal salinity" would recover more oil than otherwise, and the at "optimal salinity", the microemulsion phase diagram will resemble the phase diagram shown in FIG. 2. Thus to maximize oil recovery, it is desirable to use such optimal microemulsions. However, in many cases, an optimal microemulsion system will have too low of a viscosity for use in the formation, and viscosity is closely tied to mobility control.
Without proper mobility control the injected fluid will tend to finger through the reservoir and bypass a substantial portion of the oil. In other words, as the injected fluid travels through the reservoir between the injection wells and the production wells, it contacts less than the total volume of the reservoir within the injection well-production well pattern. The fraction of the volume of the reservoir that is swept by the injected fluid is termed the "sweep efficiency" and is expressed as a percentage of the total reservoir volume in the pattern. The sweep efficiency of a typical waterflood or surfactant waterflood may typically be less than 75 percent when the waterflood reaches its economic limit, thus one quarter or more of the reservoir may not have been contacted by the injected fluid by the end of the flooding operation.
Low flooding operation sweep efficiency is usually explained by the fact that the injected fluid has the ability to move through the reservoir at a much faster rate that the oil which it is displacing. The fingering and bypassing tendencies of the injected fluid are due in part to its relatively low viscosity.
A number of procedures have been suggested to date for improving conventional and surfactant waterflooding to reduce the degree of fingering and bypassing and to increase the sweep efficiency. These suggestions relate to the incorporation of a viscosity imparting agents, generally polymeric material, into the waterflood to increase the viscosity of the flood water, or the use of certain surfactants (see for example U.S. Pat. No. 3,753,465, issued Aug. 21, 1973).
The addition of other surfactants or other viscosifying agents to the microemulsion, while increasing the viscosity of the microemulsion, is not without its problems. As stated earlier, surfactants suffer from depletion. Some of the viscosity imparting agents have a tendency to plug the formation, some are unstable, some have relatively little thickening effect, and most do not have the ability to lower the interfacial between the oil and water to desired levels. Additionally many of the surfactants and viscosity imparting agents are quite expensive and their use is not economically feasible. In many instances, the polymers and surfactants are incompatible with each other at the optimal conditions.
However, the most important drawback of using other surfactants and viscosifying agents to control the viscosity of the optimum microemulsion, is that, the addition of these other materials to the microemulsion system, will have a tendency to change the phase behavior of the microemulsion. The system will generally change from the optimum microemulsion system as shown in FIG. 2 to either of the non-optimum systems as shown in FIGS. 1 and 3. In other instances, even if the addition of these other materials does not change the system to a non-optimum system, the addition of there viscosifying agents will many times change the single phase microemulsion to a multiphase microemulsion.
Thus there exists a need to improve the efficiency of EOR techniques utilizing microemulsions from the single phase region in the ternary diagram of an optimum microemulsion system by providing a technologically and economically feasible method of increasing the viscosity of a single phase optimum microemulsion without negatively altering the phase behavior of the system, and without shifting the system to a non-optimum system.